I'll be really straightfoward:
If she did not give you a single droplet of doubt, and asked you why did you find it "humorous" so you could explain yourself, then she's well served with the two-faces, and they'll bite her eventually for this misstep. This is the price to pay when one is hastly making assumptions and not listening to both sides of the story.
Move on, and keep the memory of your friend with you. When asked, be honest about it. Do your thing, and they'll do theirs.
This kinda situation is VERY complicated, because all attempts to get to her right now will be (mostly) fruitless.